The present invention relates to a new and improved coupling assembly which may function as either a clutch or a brake between a pair of elements.
Known coupling assemblies which can be used as either a clutch or a brake are disclosed in U.S. Pat. Nos. 3,022,877; 3,092,231; 4,190,136; and 4,795,013. Each of these known coupling assemblies has an annular rim which is fixedly connected to one of two elements between which force is to be transmitted. An annular array of friction shoes is connected with the rim and is engageable with a cylindrical friction surface on the other element. An annular fluid extensible tube is disposed between the rim and the annular array of friction shoes.
When the tube in any one of the known coupling assemblies is radially extended under the influence of fluid pressure, the friction shoes are pressed against the friction surface to engage the coupling assembly When the fluid pressure in the tube is reduced, the tube retracts and the friction shoes move out of engagement with the friction surface to disengage the coupling assembly. Therefore, when the tube is retracted, the coupling assembly is disengaged and the two elements can be freely rotated relative to each other. It is only by expanding the tube, under the influence of fluid pressure, that coupling assembly is engaged to prevent relative rotation between the two elements.
Although these known coupling assemblies have been satisfactory in their mode of operation, there are certain operating environments in which it is desirable to have failsafe engagement of the coupling assembly. Thus, in the event of a failure to inflate the fluid extensible tube in these known coupling assemblies, the coupling assembly remains disengaged. The coupling assembly can be engaged only by inflating the tube. In certain operating environments, it is desirable to have the coupling assembly engaged if there is a failure to supply fluid under pressure to the fluid extensible tube.